The present invention relates to methods and apparatus for maintaining synchronization between a control circuit and a rotor of a polyphase motor during power interruptions, particularly when rotor position sensors are not employed in the control and drive of the polyphase motor.
Polyphase AC motors, such as permanent magnet, synchronous machines must be driven such that the windings thereof are energized as a function of the rotor position (and, thus, the rotor flux) in order to obtain driving torque from the machine. Conventionally, the rotor position is obtained by way of one or more rotor position sensors within the polyphase motor assembly, which sensors provide signals indicative of the rotor position to a control circuit.
The material and labor costs associated with employing position sensors within the polyphase motor assembly are undesirable and, therefore, techniques have been developed that permit proper energization of the windings of a polyphase motor without using position sensors. Some of these techniques are discussed in, for example, U.S. Pat. Nos. 5,565,752; and 5,929,577, the entire disclosures of which are hereby incorporated by reference.
Control and drive techniques that do not require position sensors share a common characteristic, namely, that the rotor position of the polyphase motor is unknown at startup. In order to deal with the unknown rotor position, these techniques employ an open-loop acceleration process where the windings of the polyphase motor are driven without synchronization with the rotor position until the motor reaches a threshold rotational speed. At this speed, the polyphase motor generates signals of sufficient magnitudes to provide an indication of the rotor position. Among the signals that may be indicative of the rotor position are the back electromotive force (BEMF) voltages of the windings, the winding currents, etc.
Reference is now made to FIG. 1, which illustrates a block diagram of a conventional system 10 for controlling and driving a polyphase motor 18, which system measures the BEMF voltages of the polyphase motor 18 to determine rotor position. The system 10 includes a DC source 12, a control circuit 14, a driver circuit 16, and the polyphase motor 18. The DC source 12 produces a voltage, VDC, with respect to ground, which is utilized to provide an operating DC voltage, VCC, to the control circuit 14 and to provide a DC bus voltage, VBUS, to the driver circuit 16. The control circuit 14 provides commutation control signals to the driver circuit 16 such that the driver circuit 16 can properly energize the windings of the motor 18. The windings of the motor 18 (which are typically in the standard wye configuration, but which may also be in the delta configuration) are coupled to the driver circuit 16 by way of nodes A, B, and C. The driver circuit 16 provides various current paths among these nodes, the DC bus, and ground in order to drive the polyphase motor 18. The control circuit 14 monitors the voltages at nodes A, B, and C, such as the BEMF voltages, and utilizes same to maintain synchronization with the rotor position of the polyphase motor 18.
Unfortunately, the conventional techniques of monitoring signals indicative of rotor position (such as the BEMF voltages) cannot maintain synchronization with the polyphase motor 18 in the event of a power interruption, even if the power interruption is only momentary and the motor 18 has not stopped turning. This is so because during the power interruption the control circuit 14 is de-energized and looses all synchronization information. This is best seen in FIG. 2, which is a graphical representation of the characteristics of the voltage at node A, the DC bus voltage, and the DC source voltage during a power interruption. At time t0, a power interruption occurs and the DC source voltage, VDC, falls from about 24 volts to about 0 volts. Assuming that there is some impedance between the DC source 12 and the DC bus, the DC bus voltage, VBUS, (and VCC) falls after t0 as a function of the speed of the polyphase motor 18, which is decelerating. Likewise, the voltage at node A falls as a function of the slowing rotational speed of the polyphase motor 18. When the operating DC voltage, VCC, has fallen below, for example, about 15 volts, the control circuit 14 ceases to function properly and loses synchronization with the rotor position of the polyphase motor 18.
When power is restored, resynchronization of the control circuit 14 to the rotor position must be established in order to properly commutate the windings of the polyphase motor 18. Among the conventional processes for reestablishing synchronization is permitting the polyphase motor 18 to stop rotating and restarting the polyphase_motor 18 utilizing the open-loop acceleration process discussed above. This technique may be unsatisfactory for various reasons, including the delays associated with stopping and restarting the polyphase motor 18, which are exacerbated when the inertias of the motor load and/or the rotor itself are large.
Other techniques have been developed for reestablishing synchronization between the control circuit and the rotor position, which techniques are set out in detail in U.S. Pat. Nos. 5,223,772; 5,172,036; and 6,194,861, the entire disclosures of which are hereby incorporated by reference. These conventional techniques, however, all presuppose that synchronization has been lost and must be reestablished using some specialized process. The manifest disadvantage of these techniques, therefore, is the reactive approach that they take to the loss of synchronization. Indeed, they do not address the root problem: the loss of synchronization itself.
Accordingly, there are needs in the art of new methods and apparatus for maintaining synchronization between a control circuit and a rotor of a polyphase motor during power interruptions, so long as the motor is rotating.